Adaptive contention window in discontinuous wireless communication channels

ABSTRACT

A system and method for discontinuous wireless communication. The system and method include estimating a first likelihood of channel collisions at start of a discontinuous channel interval, wherein the estimated likelihood of collision is increased when a transmission failure is detected during a portion of one or more previous channel intervals; setting a size of a contention transmission window at start of a current channel interval, according to the first estimated likelihood of channel collisions; estimating a second likelihood of channel collisions for a next channel interval; and dynamically changing the size of the contention transmission window for the next channel interval, according to the second estimated likelihood of channel collisions.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems, and specifically to those where access to the communicationchannel is limited to periodic intervals. Additionally, the inventionpertains to systems where access to the communication channel is basedon a contention algorithm. One such architecture is found is the systemvariously known as Wireless Access in a Vehicular Environment (WAVE) orDedicated Short Range Communications (DSRC).

BACKGROUND

Access to a wireless communications channel may be via controlled orcontention access. In a controlled access system, such as in a cellularor WiMAX (IEEE 802.16) system, devices are generally allocated transmitopportunities that do not conflict with transmissions from otherdevices. (Some exceptions may exist, as when a device initially entersthe network.). In a contention access system, devices transmit atlocally determined times, and the possibility of collisions and dataloss exits. Examples of such systems are Ethernet (IEEE 802.3) and Wi-Fi(IEEE 802.11).

Collisions are detrimental to the operation of the network, as theyresult in increased data latency and decreased system throughput (due toretransmissions) and potential data loss. Collisions are difficult toavoid in contention access-based communication systems. Existingcommunication protocols, such as those named above, attempt to avoidcollisions. Devices operating in such systems attempt to receive fromthe medium before transmitting, and only proceed with transmission ifthe channel is found to be idle. However, this technique is noterror-proof. For example, when a device A is transmitting on the medium,if a device B decides to transmit at the same instant as device A, thetwo transmissions will collide, typically preventing other receiversfrom correctly interpreting either transmission. Even if device B'stransmission does not occur at the exact instant as device A's, delaysin the system (e.g., propagation, processing, and/or receive/transmitswitching delays) will prevent the second device from detecting atransmission that begins shortly before its own, again resulting in acollision.

Contention access systems may attempt to recover from data lost throughcollisions. In this case, when a collision is detected, the transmittingdevice waits for some time (the “backoff time”) and retransmits thedata. To guard against a repeating series of collisions, the backofftime may be randomized over a time span called the “contention window.”

The above recovery mechanism is problematic when multicast or broadcasttransmissions are used. Unlike unicast transmissions, which can benefitfrom acknowledgements or similar feedback mechanisms, multicasttransmissions typically have no feedback from the receiver to thetransmitter. Thus the transmitter may have no way to recognize theoccurrence of a collision.

The optimal contention window size depends on the number of devicesattempting to transmit on the channel. If there are few devices, a shortcontention window (CW) allows all devices to transmit with small chanceof collisions. If more devices are attempting to transmit, the optimalcontention window size is longer, to spread transmissions over a longertime and thus reduce the probability of collisions. This is illustratedin FIG. 1, where channel throughput/efficiency curves for threedifferent contention window sizes are shown. The small CW performs bestat light channel loading, and the large CW performs best at high channelload. At the lowest ends of the curves, where CW is larger than optimal,there is poor throughput performance because the channel is idle duringmuch of the contention window. At the highest ends of the curves, whereCW is smaller than optimal, channel performance is poor because thehigher number of transmission attempts in the contention window resultsadditional collisions.

Existing algorithms attempt to optimize performance by dynamicallyadjusting the CW size. For example, IEEE 802.11 uses an exponentialbackoff. The CW starts at a small value, CWmin. As long as transmissionsare successful (as determined for example, be the receipt of anacknowledgement), CW remains at the CWmin value. However, upon atransmission failure, CW is doubled in size. Another failure will causeanother doubling, until a predefined maximum value, CWmax, is reached.This increases CW size under the assumption that collisions result fromthe presence of a larger network load, and that the system will performmore efficiently with the larger CW, as described above. Any successfultransmission (including a multicast transmission, with or without acollision) causes CW to revert to the small CWmin value.

Additional refinements may exist in traditional contention schemes. Forexample, transmissions of varying priority levels may be assigneddifferent contention window sizes to provide the higher priority trafficwith a higher likelihood of transmission.

When traditional contention access techniques are applied to adiscontinuous channel, performance degradation in the form of increasedcollisions may result. A discontinuous channel is one where access islimited to periodic or semi-periodic intervals. For example, in a WAVEsystem, control and safety information is exchanged on a control orcontrol/safety information (CSI) channel during a “CSI channel interval”which occurs for example 50 ms out of every 100 ms. During the other 50ms, the “service channel interval,” devices may tune to other radiochannels for other types of communications exchanges, and no controlinformation is exchanged. Any control information arriving fortransmission during the service channel interval must be queued fortransmission during the next CSI channel interval. Likewise, servicetraffic arriving during the CSI channel interval must be queued untilthe next service channel interval. Since there is a higher than averageprobability that multiple devices have queued traffic for transmissionat the beginning of the channel interval, traditional contentiontechniques result in poorer system performance when operating overdiscontinuous channels, since the CW size must adapt over time to theincreased channel load found at the start of the channel interval.Therefore, there is a need for a more effective wireless communicationchannel, in which the contention windows can dynamically be changed atthe start of a discontinuous channel interval based on predicted channelbehavior.

SUMMARY

The present invention performs substantially better by choosing a moresuitable CW size (CWinit) to use at the start of the channel intervaland changing the size of CWinit dynamically according to predictedchannel conditions. The present invention introduces a contention accesstechnique for improving communications network performance overdiscontinuous channels via manipulation of the contention window. Thecontention window may be reset to a larger size at the start of eachchannel access interval to spread transmissions out in time during heavyloading, thus reducing the probability of collisions. The size of thecontention window can then be dynamically adjusted during the course ofthe channel interval via means known in the prior art.

In some embodiments, the present invention is a method for discontinuouswireless communication. The method includes estimating a firstlikelihood of channel collisions at start of a discontinuous channelinterval, wherein the estimated likelihood of collision is increasedwhen a transmission failure is detected during a portion of one or moreprevious channel intervals; setting a size of a contention transmissionwindow at start of a current channel interval, according to the firstestimated likelihood of channel collisions; estimating a secondlikelihood of channel collisions for a next channel interval; anddynamically changing the size of the contention transmission window forthe next channel interval, according to the second estimated likelihoodof channel collisions.

The first (and/or second) estimated likelihood of channel collision isdecreased when a transmission success is detected during a portion ofone or more previous channel intervals, and the first (and/or second)estimated likelihood of channel collision is increased when atransmission failure is detected during a portion of one or moreprevious channel intervals.

Furthermore, the size of the contention transmission window may beincreased when the first or second estimated likelihood of channelcollision is higher than a predetermined value, and the size of thecontention transmission window may be decreased when the first or secondestimated likelihood of channel collision is lower than thepredetermined value.

In some embodiments, the present invention is a device for discontinuouswireless communication. The device includes a memory for storing datafor a transmission queue; a transceiver for transmitting and receivingsignals; and a processor configured to detect a transmission failureduring a portion of one or more previous channel intervals and estimatea first likelihood of channel collisions at start of a discontinuouschannel interval, wherein the estimated likelihood of collision isincreased when a transmission failure is detected by the transceiverduring said portion of one or more previous channel intervals, to set asize of a contention transmission window at start of a current channelinterval according to the first estimated likelihood of channelcollisions, and to estimate a second likelihood of channel collisionsfor a next channel interval, wherein the transceiver transmits thestored data for the transmission queue according to the set size of thecontention transmission window.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the performance of an exemplary contention communicationschannel under varying traffic load and with different contention windowsizes.

FIG. 2 shows channel intervals aligned in time and frequency.

FIG. 3 shows message arrivals and channel loading.

FIG. 4 shows an example of a contention backoff algorithm per the priorart.

FIG. 5 shows representative performance of a traditional communicationsystem with adaptive contention window.

FIG. 6 depicts an exemplary flow chart, according to some embodiments ofthe invention.

FIG. 7 illustrates the relationship between channel behaviors in onechannel interval and the next channel interval, according to someembodiments of the invention.

FIG. 8 shows an exemplary process flow performing the transmission ofdata, according to some embodiments of the invention.

FIG. 9 shows representative performance of a communication system withan adaptive initial contention window at the beginning of a channelinterval, improved over the traditional system performance shown in FIG.5, according to some embodiments of the invention.

FIG. 10 is an exemplary diagram illustrating how conditions observed inprevious M channel intervals may be used in calculating the initialcontention window size for a current channel interval, according to someembodiments of the invention.

FIG. 11 a and FIG. 11 b are an exemplary diagrams illustrating how thesize of the initial contention window adapts to heavy and light trafficloading in recent channel intervals, according to some embodiments ofthe invention.

FIG. 12 is an exemplary hardware block diagram of a radio device,according to some embodiments of the invention.

DETAILED DESCRIPTION

FIG. 2 shows a representative timing of a typical DSRC/WAVE system.During the CSI channel interval, devices operate on the CSI channel.During the service channel interval devices may operate on one ofseveral service channels. Therefore, operation on either a servicechannel or the CSI channel is discontinuous. For example, considercontrol-type transmission units arriving at a device at times T1 and T2,while the device is operating on service channel 1. These transmissionunits are queued for transmission until the start of the next CSIchannel interval, at time T3. Control-type transmission units arrivingat other devices during the service channel interval are also queued fortransmission on the CSI channel at T3, increasing the likelihood ofcollision.

FIG. 3 illustrates an exemplary behavior of packet traffic in adiscontinuous communications environment depicting message arrivals andchannel loading. Packet arrivals to the system occur at a nominalaverage rate over time, both during and between access intervals. Duringthe no access time, packets are queued, so the transmit queue at a givendevice grows. At the beginning of the access interval, device queues areloaded, so the channel loading is high. During this time, the highertraffic load increases the potential for collisions. As packets aretransmitted during the access interval, queue sizes and channel loadingdecrease until nominal levels are reached (here shown about halfwaythrough the access interval). At this point an equilibrium state isreached which continues to the end of the access interval, at which timethe cycle repeats.

FIG. 4 illustrates a typical contention backoff process according to theprior art, such as those implemented in a device based the IEEE 802.11standard. Initially, the contention window CW is set to a predefinedminimum value CWmin (41). The device determines if a packet is queuedfor transmission (42), and if not, it waits. When a packet is ready fortransmission, it calculates the transmission time (43), which consistsof a random backoff within the contention window. At the transmissiontime, the device ensures that the medium is not in use (44), andtransmits the packet accordingly (45). If the packet is successfullytransmitted (e.g., an acknowledgement is received showing no collision),then the device goes to its initial state. If the packet is notsuccessfully transmitted (46), the device checks whether CW is at itsmaximum value (47). (With no feedback concerning its reception, thetransmission of a broadcast packet is considered a success, in whichcase the process restarts.). If not, the CW size is increased (48) andan attempt is made to retransmit the packet at a later time.

If CW has reached its maximum size, the device checks whether the packethas already been retransmitted the maximum allowed times (49). If not,another attempt is made to retransmit the packet. If the maximumtransmit attempts have been reached, the packet is dropped, and thealgorithm resumes from the beginning. (Under some conditions, forexample, if the maximum retry count is low, it is possible that themaximum transmit attempts could be reached before the contention windowreaches its maximum size. For simplicity, this case is notillustrated.).

FIG. 5 shows how the above process performs in a discontinuouscommunications system. As illustrated, at the start of the channelaccess interval, the CW is small, but the channel load is large,resulting in poor performance due to excessive collisions. It takes sometime for the CW value to adapt to the high load at the start of thechannel interval. The present invention improves this performance byadjusting CW to a value appropriate to the current channel conditions.

FIG. 6 depicts an exemplary state diagram, according to some embodimentsof the invention. There are three states as indicated in FIG. 6. Eachstate is described in detail in the following paragraphs.

-   A. Predict (estimate) channel behavior (load). This state involves    predicting the likelihood of multiple transmissions contending for    channel access at the start of a channel access period, based on    recent channel activity, historical activity, and/or other relevant    information. For example, multiple transmissions contending for    channel access at the start of a channel access period constitute a    greater chance of collision.-   B. Calculate the initial Contention Window size (CWinit). Based on    the estimated (predicted) channel behavior (load), a preferred    CWinit is calculated, to optimize system performance under the    estimated (predicted) channel conditions. For example, a larger    CWinit reduces the likelihood of collision.-   C. Perform contention access. In this state, the calculated CWinit    is used in the contention access process, starting in the next    channel interval. Using the selected CWinit value, improved channel    performance is achieved.

Predict (estimate) channel behavior (load) involves predicting thelikelihood of multiple transmissions contending for channel access atthe start of a channel access period, based on recent channelactivities, historical activities, and/or other information. Forexample, there are several observable events that can be used to predictchannel behavior in regards to the transmissions attempted at thebeginning of a channel interval. Transmission behavior in the previouschannel interval N−1 is typically a good predictor of behavior in theupcoming interval N. This is because traffic load changes slowlyrelative to channel intervals. Transmission behaviors in the previouschannel intervals N−2 . . . N−k may also be good predictors of behaviorin the upcoming interval N, as illustrated in FIG. 10.

As described earlier, transmission queues collect packets fortransmission leading up to the start of the channel interval, resultingin a bottleneck and higher probability of collisions at the beginning ofthe channel interval. Over the course of the channel interval, packetsare successfully delivered, easing the congestion and resulting inrelatively fewer collisions. For this reason, the channel behavior inthe early periods near the beginning of the channel interval, before thequeues reach steady state (at about 50% of the channel intervalillustrated in FIG. 3) is used to estimate the channel loading for thepurpose of contention window calculation. The time when the queues arebacklogged, before they reach steady state, is the time that the CW isapproaching its optimal size for the current conditions. The longer ittakes to achieve steady state, the more a higher value of CWinit isindicated.

Events in the early period of interval N−1 that may be used to estimatechannel load and are strong predictors of the likelihood of collisionsin the early period of interval N may include the following.

-   -   The communicating device having had an unsuccessful or deferred        transmission attempt in the early period of interval N−1 (and        intervals N−2 . . . N−k) implies a higher likelihood of        collision in interval N. In contrast, the communicating device        having had a successful transmission attempt in the early period        of interval N−1 (and intervals N−2 . . . N−k) implies a lower        likelihood of collision in interval N.    -   Detecting collisions by the communicating device among other        devices in the early period of interval N−1 (and intervals N−2 .        . . N−k) implies a higher likelihood of collision in interval N.        The fewer the detected collisions, the less likely collisions in        interval N.    -   Detecting channel busy by the communicating device during a        large fraction of the early period of interval N−1 (and        intervals N−2 . . . N−k) implies a higher likelihood of        collision in interval N. The lower the channel busy fraction,        the less likely collisions in interval N.

The same predictors described above, considered across the latterportions of interval N−1, rather than just the early period, can beconsidered weak predictors of the likelihood of collision in interval Nas shown in FIG. 7.

There may be other predictors available in a specific system. The abovepredictors (factors) can be weighted differently when used in predictingthe channel behavior. For example, collisions and/or being busy ininterval N−1 may be given a higher weight than collisions and/or beingbusy in interval N−2, etc. Similarly, collisions in interval N−1 may begiven a higher weight than being busy in interval N−1, etc. In otherwords, the present invention allows a higher weight factor for atransmission failure detected during an immediate previous channelinterval and factors increasingly lower weight factors to a transmissionfailure detected during older previous channel intervals.

An exemplary formula for calculating the initial contention window sizeis described below.

${CWinit}_{N} = {{CW}_{0} + {s{\sum\limits_{n = 1}^{k}\;\left( {L_{n}*{w(n)}} \right)}}}$

-   -   a) where:    -   b) CWinit_(N) is the initial contention window size for the        current channel interval N    -   c) CW₀ is a base, or minimum allowed contention window size,        used for example when there is no channel loading    -   d) s is a scaling factor to keep the range of contention window        sizes within a predefined bound. (For example, the contention        window size must not exceed the channel interval duration.)    -   e) k is the number of recent channel intervals that are        considered in the calculation. In a simple case, k=1 and only        the most recent channel interval is considered.    -   f) n is the count of the recent channel intervals. For example,        n=1 indicates the immediately previous channel interval (N−1);        n=k indicates the oldest channel interval that is considered in        the calculation.    -   g) L_(n) is the loading factor (predictor of collisions)        calculated for channel interval n. Calculation of the loading        factor may consider such factors as collisions or channel busy        as described previously. In a simple case, L_(n) takes only two        values, e.g., 0 for light loading, 1 for heavy loading.    -   h) w(n) is a function of the remoteness in time of the channel        interval for which L_(n) was calculated. w(n) in this example is        a positive number and has an inverse relationship to n (w(n)        decreases as n increases). Its effect is to weight L_(n) such        that more recent loading factors have a greater impact on the        result. In the simplest case, w(n) equals 1.

In some embodiments, a communication device according to the presentinvention monitors the events listed above, and generates an estimate ofwhether collisions are likely in interval N. The weight given to eachevent in the estimation process may be tailored to the specificcharacteristics of the operational system. Strong predictors are givenmore weight than weak predictors. Note that events over additionalprevious channel intervals (N−2, N−3, etc.) may be used to refine theestimate.

Calculating the initial Contention Window size is performed based on theestimated behavior to optimize system performance under the estimatedchannel conditions.

More specifically, if a high likelihood of collisions in interval N isdetermined, the initial contention window size (CWinit) is increased asillustrated in FIG. 11 a. If there is a low likelihood of collisions ininterval N, then CWinit is decreased as illustrated in FIG. 11 b.

In the Perform contention access state of FIG. 6, the calculated CWinitis used for transmission, starting in the next channel interval N.

FIG. 8 shows an exemplary process flow performing the transmission ofdata, according to some embodiments of the invention. At the end of anaccess interval (81), packets arriving for transmission are queued (82),and none are transmitted. Upon the start of the next access interval(83), CW is set to a new value, CWinit (84), which is used in thecalculation of the first packet transmission time (843). Also, when atransmission time is calculated, the device determines whether thecalculated time falls within the current access interval (85). If it isthe end of access interval, the device is done transmitting in thecurrent access interval, and begins queuing packets (82) and waiting forthe next access interval (83). At the start of the next interval, thenext transmit time is calculated (843), the normal transmit sequencefollowed (844-845), and CW is updated (846-849). Note that a broadcasttransmission may be considered a success by default in block 846, inwhich case, CW is reset to CWmin and the process repeats.

FIG. 9 shows how the invention performs in a discontinuous communicationsystem, according to some embodiments of the invention. As shown, at thestart of the channel access interval, the CW is better suited for thehigher traffic load, resulting in fewer collisions and better efficiencycompared to the traditional algorithm illustrated in FIG. 5. CW can morequickly adapt to the optimal value, and the queues reach steady staterelatively quickly, resulting in overall improved system throughput andreduced latency.

The above discussion provides a description of a distributed real timeembodiment of the invention. Other embodiments include a centralizedreal time embodiment and an offline embodiment. In these embodiments,channel behavior information is collected (by a central processing unit,or an offline processing unit) which performs the steps of channel loadestimation and CWinit calculation. The CWinit value is then distributed(e, g., via a broadcast control message or other configurationmechanism), for use by the communicating devices. In the offline case,the central processor may consider historical behavior information,e.g., accommodating a spike in traffic during morning rush hour.

FIG. 12 is an exemplary hardware block diagram of a radio device,according to some embodiments of the invention. The transceiver (1201)includes but not limited to such standard components such as modulator,demodulator, and amplifier. It also may include a channel sensingmechanism (e.g., received power envelope detector) to detect a channelbusy state. The modulator function may also include function to detectchannel collisions. The transceiver accepts packets for transmission,and delivers information (e.g., busy status) about the medium state tothe processor (1202).

The processor may include one or more hardware or software processingelements. Its functions include preparing packets for transmission andimplementing the contention access algorithms, including the setting ofthe contention window size. It considers the medium state as describedin this document in deciding when to send packets from transmit queue(1204) residing in the memory (1203), to the transceiver fortransmission.

Note that the present invention may be applied to multi-queue (e.g.,priority-based) systems as well. In the case of a multi-queue system, adifferent CWinit value can be used for different transmission prioritylevels.

It will be recognized by those skilled in the art that variousmodifications may be made to the illustrated and other embodiments ofthe invention described above, without departing from the broadinventive scope thereof. It will be understood therefore that theinvention is not limited to the particular embodiments or arrangementsdisclosed, but is rather intended to cover any changes, adaptations ormodifications which are within the scope and spirit of the invention asdefined by the appended claims.

The invention claimed is:
 1. A method for wireless communication betweena transmitting device and a receiving device over a discontinuouschannel, the method comprising: estimating a first likelihood of channelcollisions at start of a first discontinuous channel access interval,wherein channel collisions are based on transmission failures ofinformation transmitted by the transmitting device in the discontinuouschannel, wherein the estimated likelihood of channel collisions is setto be greater than a likelihood of channel collisions that was set for aprevious discontinuous channel access interval, when a transmissionfailure of the information transmitted by the transmitting device in thediscontinuous channel is detected during a portion of one or moreprevious channel access intervals, and wherein the discontinuous channelincludes a no access interval during which the channel is not availablefor data transmission and an access interval during which the channel isavailable for data transmission; setting a size of a contentiontransmission window at start of a current discontinuous channel accessinterval after the first discontinuous channel access interval,according to the first estimated likelihood of channel collisions;estimating a second likelihood of channel collisions for a nextdiscontinuous channel access interval after the current discontinuouschannel access interval; and dynamically changing the size of thecontention transmission window for the next discontinuous channel accessinterval, according to the second estimated likelihood of channelcollisions, where the first or second estimated likelihood of channelcollision is increased when a channel busy is detected during a portionof one or more previous channel intervals.
 2. The method of claim 1,wherein the first estimated likelihood of channel collision is decreasedwhen a transmission success is detected during a portion of one or moreprevious channel access intervals.
 3. The method of claim 2, wherein thesize of the contention transmission window is decreased when the firstor second estimated likelihood of channel collision is lower than apredetermined value.
 4. The method of claim 1, wherein the secondestimated likelihood of channel collision is decreased when atransmission success is detected during a portion of one or moreprevious channel access intervals.
 5. The method of claim 1, wherein thesecond estimated likelihood of channel collision is increased when atransmission failure is detected by the receiving device during aportion of one or more previous channel access intervals.
 6. The methodof claim 1, wherein the first or second estimated likelihood of channelcollision is adjusted based on detection of channel busy and queue sizeduring a portion of one or more previous channel access intervals. 7.The method of claim 1, wherein the size of the contention transmissionwindow is increased when the first or second estimated likelihood ofchannel collision is higher than a predetermined value.
 8. The method ofclaim 1, wherein an initial sized contention window is used at thebeginning of each channel access interval.
 9. The method of claim 1,wherein setting the size of the contention transmission window at startof a current discontinuous channel access interval comprises setting adifferent size for the contention transmission window for transmission,according, to different priority levels of multiple data queued fortransmission.
 10. The method of claim 1, wherein said estimating firstor second likelihood of channel collisions comprises of predictinglikelihood of multiple transmissions contending for channel access,based on recent or historical channel activities, and increasing saidfirst or second estimated likelihood of channel collisions when multipletransmissions are contending for channel access at the start of achannel access period.
 11. The method of claim 1, wherein the first orsecond estimated likelihood of channel collision is increased when atransmission failure by any one of a plurality of communicating devicesis detected during a portion of one or more previous channel accessintervals.
 12. The method of claim 1 further comprising factoring ahigher weight factor for a transmission failure detected during animmediate previous channel access interval and factoring increasinglylower weight factors to a transmission failure detected during previouschannel access intervals older than the immediate previous channelaccess interval.
 13. The method of claim 1, wherein the portion of oneor more previous channel access intervals are the first 50% of channelaccess intervals of one or more previous channel access intervals.
 14. Adevice for discontinuous wireless communication comprising: a memory forstoring data for a transmission queue; a transceiver for transmittingand receiving data; and a processor configured to detect a transmissionfailure during a portion of one or more previous discontinuous channelaccess intervals and estimate a first likelihood of channel collisionsat start of a first discontinuous channel access interval, whereinchannel collisions are based on transmission failures of informationtransmitted by the device in a discontinuous channel, wherein theestimated likelihood of collision is set to be greater than a likelihoodof channel collisions that was set for a previous discontinuous channelaccess interval, when a transmission failure of the informationtransmitted by the device in the discontinuous channel is detectedduring said portion of one or more previous channel access intervals, toset a size of a contention transmission window at start of a currentdiscontinuous channel access interval after the first discontinuouschannel access interval according to the first estimated likelihood ofchannel collisions, and to estimate a second likelihood of channelcollisions for a next discontinuous channel access interval after thecurrent discontinuous channel access interval, wherein the transceivertransmits the stored data for the transmission queue according to theset size of the contention transmission window, and wherein thediscontinuous channel includes a no access interval during which thechannel is not available for data transmission and an access intervalduring which the channel is available for data transmission, where thefirst or second estimated likelihood of channel collision is increasedwhen a channel busy is detected during a portion of one or more previouschannel intervals.
 15. The device of claim 14, wherein the processor isfurther configured to decrease the first estimated likelihood of channelcollision when a transmission success is detected during a portion ofone or more previous channel access intervals, and to increase the firstestimated likelihood of channel collision when a transmission failure isdetected during a portion of one or more previous channel accessintervals.
 16. The device of claim 14, wherein the processor is furtherconfigured to decrease the second estimated likelihood of channelcollision when a transmission success is detected during a portion ofone or more previous channel access intervals, and to increase thesecond estimated likelihood of channel collision when a transmissionfailure is detected during a portion of one or more previous channelaccess intervals.
 17. The device of claim 14, wherein the processor isfurther configured to increase the size of the contention transmissionwindow when the first or second estimated likelihood of channelcollision is higher than a predetermined value, and to decrease the sizeof the contention transmission window when the first or second estimatedlikelihood of channel collision is lower than said predetermined value.18. A system for wireless communication between a transmitting deviceand a receiving device over a discontinuous communication channelcomprising: means for estimating a first likelihood of channelcollisions at start of a first discontinuous channel access interval,wherein channel collisions are based on transmission failures ofinformation transmitted by the transmitting device in the discontinuouschannel, wherein the estimated likelihood of channel collisions is setto be greater than a likelihood of channel collisions that was set for aprevious discontinuous channel access interval, when a transmissionfailure of information transmitted by the transmitting device in thediscontinuous channel is detected during a portion of one or moreprevious channel access intervals, and wherein the discontinuous channelincludes a no access interval during which the channel is not availablefor data transmission and an access interval during which the channel isavailable for data transmission; means for setting a size of acontention transmission window at start of a current discontinuouschannel access interval after the first discontinuous channel accessinterval, according to the first estimated likelihood of channelcollisions; means for estimating a second likelihood of channelcollisions ‘/’ for a next discontinuous channel access interval alterthe current discontinuous channel access interval; and means fordynamically changing the size of the contention transmission window forthe next discontinuous channel access interval, according to the secondestimated likelihood of channel collisions, where the first or secondestimated likelihood of channel collision is increased when a channelbusy is detected during a portion of one or more previous channelintervals.